Gas species typically have unique spectral features in the midwave (3-5 micron) and/or the longwave (8-14 micron) infrared (IR) regions of the electromagnetic spectrum. Whether the spectral features manifest themselves as an emission spectrum or an absorption spectrum depends upon the temperature of the gas relative to its background. If the gas is cooler than the background, the gas exhibits an absorption spectrum. If the gas is warmer than the background, the gas exhibits an emission spectrum with features mirroring the wavelengths of its absorption spectrum. In either case, distinctive features may usually be found that characterize a particular gas. Systems that are tuned to detect the presence or absence of one or more spectral features of the gas so that the particular gas may be identified are known in the art.
A laser system called differential absorption lidar (DIAL) has been effectively used to detect and identify the presence of specific target gas species. The sensitivity of the DIAL system is achieved by performing a comparison between an “on-line” spectral band (that corresponds in wavelength to an absorption feature of the target gas species) and a nearby “off-line” spectral band (that corresponds in wavelength to a non-absorption feature of the target gas species).
DIAL systems, however, tend to be relatively expensive and require active illumination. A less expensive system is, therefore, desirable and would open the door to a variety of new applications. Such a system may include a small portable unit used to detect the presence of harmful gases on a battlefield or at an airport, for example. Another system may be useful in homeland security surveillance, such as at airports, subways, bus stations, naval yard, malls, government buildings, etc.
Furthermore, DIAL systems require active illumination from a laser source. Active illumination is undesirable, since the source of the illumination may potentially be detected by an enemy.
Various non-laser IR systems are known for detecting and measuring characteristics of a gas. These systems are also based on an active illumination source that transmits light through a gas. One such system is described in U.S. Pat. No. 7,141,793, which is incorporated herein by reference in its entirety. An example of such a system, designated as 8, is illustrated in FIG. 1 and briefly described below.
The system 8 includes beam 2 from a radiation source 1, which passes through emission plume 3. The beam 2 then passes through a lens 4, a filter wheel 5 and is subsequently detected by a detector 6. The concentrations of various components of the emission plume may be calculated by determining the amount of radiation of certain characteristic wavelengths which have been absorbed from the beam upon its passage through the emission plume.
The system 8 includes a filter wheel 5. As disclosed in the aforementioned patent, the filter wheel may include a set of filters distributed on the filter wheel. The filters on the wheel are rotated so that one filter at a time is aligned with a single detector, such as detector 6. The single detector is assumed to have a sufficiently broad frequency response so that it may cover all of the detection bands of interest. Each of the filters on the wheel allow transmission of only certain wavelengths through the filter wheel. Thus, each of the filters is used to isolate different detection bands of radiation for detection of different components of the emission plume.
In some situations, a sample of the gas plume must be captured in a gas cell. These gas cells typically allow for only a single point measurement. The aforementioned systems, whether they include a gas cell or not, are relatively insensitive. Therefore, these systems require external illumination through the emission plume. Thus, these systems include active sources of illumination, which are undesirable.
The present invention, as will be explained, increases sensitivity of a non-laser gas detector by using a DIAL-like methodology for detecting, measuring and identifying gas plumes. The present invention uses multiple filters, at least one of which is tuned to a spectral characteristic of the target gas (on-line) and at least one of which is tuned to a nearby wavelength in which no absorption occurs in the target gas (off-line).
By using an on-line and off-line filter combination together with subsequent processing, greater sensitivity is achieved by the present invention. As a result, the present invention facilitates detection and measurement of a gas without the requirement of an active illumination source. In fact, the present invention passively samples infrared signatures of the emission plume by using a detector array that produces a spatially-variant result, represented as an image.